[alsa-devel] Incorrect sbits for snd-usb-audio

[Sander Jansen]

Hi,

I have a Focusrite Scarlett 2i4 USB Audio Interface. The audio format
exposed by ALSA for  this device is S32_LE. From the hardware specs and
lsusb. it supposed to only have a 24 bit resolution (with the LSB bits set
to 0):

AudioStreaming Interface Descriptor:
        bLength                 6
        bDescriptorType        36
        bDescriptorSubtype      2 (FORMAT_TYPE)
        bFormatType             1 (FORMAT_TYPE_I)
        bSubslotSize            4
        bBitResolution         24

When I query the sbits for this device, I expect it to return 24, but
instead it returns 32. Looking through the usb audio driver code, it
doesn't look like this information is actually exposed to userspace. The
'parse_audio_format_i_type'  in usb/format.c merely returns the supported
formats, but doesn't include any msbits info. (and it doesn't look like it
can be easily changed to return the msbits info as well).

Was this intentionally left out, or is this a missing feature in the driver?

Thanks,

Sander